In commonly owned U.S. Pat. Nos. 3,559,669, 3,580,112, 3,593,599 and 3,610,070 there has been described a transmission system of this type wherein an output shaft, such as a traction-wheel shaft of a vehicle, is driven from a fuel-powered engine with a variable torque ratio via a set of hydraulic coupling elements, namely clutches and brakes (collectively referred to as drive-establishing means), which are shiftable under fluid pressure to provide different torque ratios. The selection of any of these torque ratios, normally referred to as "first gear", "second gear" and "third gear", is controlled by fluid-actuatable switchover means in the form of a 1-2 shift valve and a 2-3 shift valve responsive to two oppositely effective fluid pressures. One of these fluid pressures, varying generally proportionally with the speed of the output shaft, emanates from a first pressure modulator in the form of a speed-responsive pressure regulator and tends to actuate the switchover means in an upshifting sense against a biasing spring force supplemented by the other fluid pressure. This latter pressure, derived from an engine-driven pump, is regulated by a master valve so as to vary with load, as described in U.S. Pat. No. 3,593,599; for this purpose, a throttle valve controlled by a manually operable accelerator (such as a vehicular gas pedal) is inserted in a hydraulic feedback loop of the master valve so as to translate a progressive displacement of the accelerator from a retracted position to an advanced position into a correspondingly increasing fluid pressure at an outlet of the master valve acting as a second pressure modulator. This load-dependent fluid pressure is delivered on the one hand to the shift valves, for the purpose of resisting upshifting, and on the other hand to the hydraulic clutches and brakes. As the accelerator advances from its retracted position past a predetermined intermediate position, i.e. in an upper part of the load range, the throttle valve -- acting as a third pressure modulator -- places the shift valves under additional pressure to create a hysteresis effect which under given load conditions causes upshifting (e.g. from first to second gear) to take place at a higher output-shaft speed then downshifting (e.g. from second to first gear). The term "load" as used in this context refers to the displacement of the accelerator, or of a valve piston operatively coupled therewith, from the normal retracted position considered as zero load.
In this prior system, once the accelerator has advanced sufficiently to create the hysteresis effect, the fluid pressure of the second modulator increases proportionally with load so that upshifting as well as downshifting occurs at progressively higher speeds as the accelerator moves to full-throttle position or therebeyond into the so-called kick-down range. This is desirable since, from the viewpoints of fuel economy and passenger comfort, frequent gear shifts under low-load conditions (e.g. when driving on a level road) should be avoided. On the other hand, a fluid pressure varying substantially linearly with load is not the optimum actuating pressure for the hydraulic clutches and brakes which should be held operated under a pressure corresponding more nearly to the engine torque to be transmitted or absorbed. In the case of an internal-combustion engine, the rise of this engine torque with increasing fuel supply is relatively steep in a lower part of the load range but tapers off asymptotically in an upper part of that range.